Recent studies indicate that the pathogenesis of cholesterol cholelithiasis is not explained simply by differences in cholesterol solubility in mixed bile acid/phospholipid micelles of different composition. Two observations support this conclusion: 1) bile is frequently supersaturated with respect to cholesterol in normal control subjects; and 2) despite similar degrees of supersaturation, nucleation time is substantially shorter in bile from patients with cholesterol gallstones than from controls. Although there is a delay in movement of a supersaturated 'model bile' to equilibrium, metastability is lost in only a matter of hours rather than of days as found for normal human biles. Our preliminary evidence suggests that the "factor" producing the enhanced metastability of native bile is present in the biliary proteins. Given these premises, the present proposal is aimed at developing a base of new information on the structural and physical aspects of the biological protein-micellar lipid complex. Structural studies on the purified protein-micellar lipid complexes from native 'model' and recombinant biles will be performed by QLS, XSAS, SE, DR, DD, analytical column chromatography and other methods. Using these techniques, molecular weights will be obtained as well as estimates of diffusion coefficients and size. Physical measurements in solvents of differing sodium halide composition will permit evaluation of the contribution of preferential electrolyte interactions in the protein-micellar lipid complex compared with the 'model bile' mixed micelle. In addition, the quantitative contribution of the biliary proteins for the transport of cholesterol will be examined. The enhancement by bile proteins of cholesterol solubility/metastability in model systems mandates a thorough biochemical and immunochemical analysis of human biliary proteins. The focus of this latter study will be to define systematic differences between the protein from normal and abnormal biles that could be critical determinants defining a difference between health and disease. Then, structural characteristics that correlate with presence or absence of such proteins will be determined.